In the continuous vertical casting of metals, the molten metal is poured continuously into a mold with open top and bottom in which the metal solidifies superficially to form a continuous pseudo-ingot solid enough to be drawn from the bottom of the mold. Superficial solidification of the molten metal results from the thorough cooling which it undergoes upon contact with the wall of the mold, which is strongly cooled by a flow of water.
Up to now, graphite-lined copper molds were mostly used for the continuous casting of copper or copper-base metals. The lining wears out very quickly and cannot be re-used when the casting operation has to be interrupted for any reason.
In addition, copper molds have been used for the continous casting of large shapes, such as cakes of copper and copper-base alloys. This process has the drawback that the thin solidified metal skin tends to stick to the mold walls, and this still very weak skin is subject to being torn under the pulling strain to which it is subjected from the lower part of the ingot when drawn out of the mold by the extraction mechanism.
Copper molds are also of current use in continuous steel casting. In this case, the ingot may not only be torn as described above with respect to continuous copper casting, but there are also fusible copper particles (from the mold) which are ripped off by the steel and are dissolved and diffused into the steel; such particles make the ingot brittle at red heat, and cracks are produced.
In order to avoid the drawbacks of the abovementioned molds, molybdenum molds or molybdenum-lined molds have already been suggested, particularly for continuous steel casting. However, since molybdenum is brittle at room temperature, such molds are not only hard to machine but are in addition subject to cracking upon contact on the one hand with the ingot submitted to traction and on the other hand with the cooling fluid or the cold outside shell of the mold (in the case of a lining).